Development Flashcards
Stem cell characteristics
Endless asymmetric division (–> 1 diff cell and 1 stem cell),
- self-renewal
- daughter cells can be either stem or diff cell
- – decided by environmental OR “divisional” (genetic) cues
Self-renewal
Ability of a cell to proliferate in the same state
Environmental asymmetry of stem cells
When the environment of the daughter cells determines which differentiates and which is 2nd stem cell
Divisional asymmetry
When presence/absence of specific RNA or proteins determines which daughter cell is the stem cell and which differentiates
(both still have same DNA)
Progenitor cell
A committed, “transit” amplifying cell
(From stem cell, partially differentiated, but not final product)
- limited capacity to divide
Totipotent stem cells
from Zygote only,
Can differentiate into ANY cell of body
– even trophoblasts (placenta cells)!
Pluripotent stem cells
Can differentiate into anything except trophoblasts
*Aka: embryonic stem cells
– from inner cell mass of blastocyst
Multipotent stem cell
Can differentiate into a limited number of adult cell types
(often in specific tissue niche, ie: skin, intestine/gut, etc.)
ie: hematopoeitic stem cells (make the various blood cells)
Unipotent stem cell
Can only differentiate into a single cell type
Stem cell niche
Areas in adult body where stem cells produce daughter cells,
Secrete paracrine factors that prevent differentiation until after leave niche.
– to protect the stem cells AND limit differentiation (in niche)
ie: crypt in intestine (BUT not always a physical pit)
MSC (mesenchymal stem cell)
multipotent stem cell,
in adult bone marrow, adipose tissue, dental pulp, breast milk, intestine, etc.
– can give rise to multiple tissue types.
= source of stem cells for research.
(potential) sources of stem cells
- mesenchymal stem cells (ie: from bone marrow)
- amniotic epithelial cells
- Fetal stem cells
- umbilical cord stem cells
embryonic stem cells
can generate ANY cell type (except trophoblasts).
how? - extra accessible chromatin structure
- DNA methylation pattern
- express MANY surface receptors, all at low level
(can respond to most signals)
regulatory factors for embyronic stem cells
“master regulators” = Oct4, Sox2, Nanog;
activate genes for self-renewal,
repress genes for specific (differentiated) pathways
iPSC (induced pluripotent stem cell)
de-differentiated nucleus,
treated with 4 TFs to convert adult cell back into pluripotent stem cell
(TFs: c-Myc, Sox2, Klf4 and Oct-3/4)
HOX genes
encode transcription factors that regulate specification of cell identity;
- -> organize the body along anterior-posterior axis.
- regulation mech: unknown, but Retinoic acid = teratogen
regulation of Hox gene expression
largely unknown, but do know:
- “posterior prevalence phenomenon” - post. genes negatively regulate anterior genes
- chromatin structure
- miRNAs negatively regulate Hox expression (“complimentary” localization –> where miRNA is, Hox gets degraded)
induction
a response by a group of cells (responder tissue) to a signal from a different group of cells (inducer tissue).
–> responsible for patterns of differentiation and dvpt in embryo,
often occur in series/cascade
Organs induced by TGF-beta
- kidneys, eyes, and skeleton (BMP7)
- heart (BMP2)
- spermatogenesis (BMP8)
- limb deformations if improper signaling here
mechanism of TGF-beta induction
TGF-beta —-> phosphorylate and dimerize SMADs
(active SMADs –> regulate transcription)
- ie: SMAD 4
morphogenesis
how organs and tissues are developed (embryologically); relies on 3 factors: 1. cell adhesion 2. cell migration 3. apoptosis
mechanism of Shh induction
Sonic Hedgehog
indirectly activates transcription;
Shh binds to (INactivates) “Patched” receptor –> active “smoothened” inhibits degraders of Gli3;
==> active Gli3 into nucleus (increase trans.)
(NO Shh: active patched–> inhibit smoothened –> degrade Gli3)
diseases associated with Shh pathway mutations
- Grieg cephalopolysyndactyly (incomplete Gli3) * more severe*
- Pallister Hall Syndrome (Gli3 only represses)
- Cyclopia (mutated Shh)
- Gorlin’s Syndrome (missing Patched Receptors)
- -> overactive smoothened (not inhibited by patched)
induction by FGF
binds to tyrosine receptor kinase –> activate MAP k –> regulate transcription;
* esp. for bone development*
Associated diseases:
achondroplasia and thanatophoric dysplasia
Wnt signal pathway
= secreted glycoproteins,
stabilize beta-catenin –> transcription regulation.
(otherwise, b-catenin = degraded by APC and GSK3)
3 domains of classical cadherins
- Extracellular - binds other cadherins (also: Ca2+ bind domain)
- -> cadherins degraded if no Ca bound.
- transmembrane - localizes the protein to the plasma membrane
- intercellular - binds alpha and beta catenins (link to actin cytoskeleton)
Rho, Rac and cdc42
Ras-related molecs,
modulate protrusion from cell (ie: lamelopodia/filopodia)
by altering actin cytoskeleton.
stress fibers
contractile bundle of actin filaments with myosin II;
- -> used for cell mvmt
- stress fibers extend from ECM, contract to pull cell forward
(or) - extend past normal length –> cell springs forward bc of tension
3 forms of guidance for cell mvmt
extracellular clues that direct migration
- Haptotaxis - based on changes in surface adhesion
- Specific substance - ~haptotaxis, but for specific substance
(ie: collagen vs. other) - Chemotaxis - attracted/repelled by gradient of a diffused substance
why use xenopus as model for development
extra-utero development, so easy to watch and study
closest model organism for dvpt
mouse has closest genetic basis of development
synteny
organization (order) of genes on chromosomes in conserved between mice and humans (also C. elegans)
ex: (drosophila –> mouse –> human)
- Pair-rule gene –> Pax1 (undulated mouse)
- also: Pax3 (splotch mouse –> Waardenburg syndrome)
Master transcription factor
(if turn a cell on)
will cause determination of the cell
ex: Pax-6 (eye cells –> ectopic eye), MyoD (muscle formation)
ie: will become muscle cell
homeotic transformation
case where Hox gene for one region is missing (ie: degraded)
–> the Hox region just anterior (or posterior) will fill in that empty zone.
==> get change in body organization
(significant mutations, ie: extra limbs)
consequences of cadherin knockout
- no cell-cell adhesion
- blurred boundaries between tissue layers
- neurons lose contact w/ target
mech. of cadherin adhesion
= homophilic adhesion (btwn cells, bind by connections btwn cadherin from each cell)
*need Ca2+ and catenins (bind to actin cytoskeleton) to give strength to attachment.
diseases from cadherin mutations
- Pemphigous vulgaris: severe blistering (desmosomal cadherin)
- Usher syndrome: hereditary deafness (protocadherin –> missing/disorganized stereocilia of inner ear)
- tumor progression (general): metastasis (E cadherin)
IgCAMs
type of cell adhesion molec, weaker than cadherins,
homo- OR heterophilic;
use ankyrin and syntrophin to anchor to cytoskeleton.
* esp. for ECM and neuronal growth (axon pathways)
Integrins
type of adhesion molec; not very strong BUT “velcro principle!”
heterophilic – bind to other molecs (NOT adhesion molecs);
use Talin to link to cytoskeleton.
—> by attaching to substratum, provide force against which can move! (for cell migration) * esp. link to ECM
Components of extracellular matrix (“ECM”)
fibronectin and laminin,
collagen,
proteoglycans, etc.
piebaldism
failure of melanocytes to properly migrate from neural crest.
* unique coloration pattern!
Hirschsprung’s disease
absence of ganglia that regulate peristalsis
problem w/ neural crest cell migration
Slit chemotaxic cue
Repulsive chemotaxic cue molec, (for directing cell migration)
receptor = Robo;
ie: organizing axon crossing at midline (creates spaces btwn connections)
Netrin chemotaxic cue
attractive OR repulsive chemotaxic cue
(for directing cell migration),
–> depends on the tissue/cell type whether attracted or repulsed
(have either the attracted receptor or the repulsed receptor)
most birth defects occur…
during organogenesis
first 12 weeks
Characteristics of apoptosis
- Cytoplasm and nucleus shrink
- DNA condenses and fragments
- Breaks into small, membrane-bound bodies
Why need apoptosis in development?
+ important for structure formation, ie:
- “sculpting” (ie: apoptose webbing btwn fingers)
- tube formation
a) apoptose from middle to create lumen
b) wrap around, then apoptose thick overlapping edge - remove unwanted structures (ie: mullerian duct for male dvpt)
- regulate cell numbers (ie: pruning neurons)
2 mechanisms of apoptosis
- Suicide/intrinsic pathway (via mitochondria)
2. murder/extrinsic pathway
mode of organ fine-tuning
mesenchyme determines fate of epithelium through interaction
(specify regional cell fate)
ie: lung dvpt
Major TFs for each direction of limb development
- FGF8: proximal - distal (induced by fgf10)
- Shh: anterior - posterior
- Wnt7a: dorsal - ventral
Holt-Oram syndrome
heterozygous mutation in Tbx5,
causes mutations in FORElimb development.
(also in heart, not in hindlimbs)
2 models for mesenchyme specification of position on proximal/distal axis
- progress zone model: with time, cells become specific to more distal positioning
- Early allocation/progenitor expansion model: from early on, cells are specified into regions; each region proliferates with time
(within positional cluster)
Signaling pathway that determines DORSAL limb development
Wnt7a (expressed only in dorsal side)
also: Lmx1 (TF) is activated by Wnt7a
dorsal-specific, in mesenchyme
How the 3 axes of limb development are related
- need FGF8 (proximal/distal) to induce Shh (ant/post)
- need wnt7a (dorsal/ventral) to maintain Shh (ant/post)
*3. FGF10 ONLY at dorsal/ventral border, triggers AER formation
(–> ant/post growth linked to dorsal/ventral pattern)
